1. Field of the Invention
The present invention relates to a display device and, more particularly, to a light emitting diode (LED) and a method for fabricating the same.
2. Discussion of the Related Art
In general, when injected electrons are recombined with pores in an LED, it emits overengergy in a form of light. For example, there are a red LED using GaAsP, a green LED using GaP, and a blue LED using an InGaN/AlGaN double hetero structure.
An LED, in which voltage is low and power consumption is low, is widely used in numeral and letter indicating device, signal light sensor, light source for optical coupler, etc.
There are four requirements to fabricate an LED. They are a satisfactory luminance, a sufficient life span, thermal stability, and performance in low voltage.
Since luminance is intimately associated with a material which an LED is made of, active research and development has been directed to good materials for LEDs. Diode's life span, thermal stability, and performance in low voltage affect contact resistance with a compound semiconductor and electrodes of LEDs. Thus, methods for fabricating an LED are currently suggested in which contact resistance is lowered so that current flows smoothly around contact areas of electrodes and the compound semiconductor. It however is not easy to reduce contact resistance.
FIGS. 1A-1C is a cross-sectional view showing a structure of a background LED made of a material of a GaN type which is good for blue LEDs.
As shown in FIGS. 1A-1C, a material for electrode is deposited on a GaN layer having a doping level of less than 10.sup.15, and then annealed so that carrier dopant is accumulated at the interface of the GaN layer and the material for electrode. Accordingly, potential barrier is decreased and resistance is thus lowered.
Because the GaN has a high resistance, a material for electrtode that contacts the GaN should be carefully selected to lower contact resistance. Titanium is mainly used as a material for electrode because TiN is formed at the interface of the titanium and a GaN layer.
That is, when N in the GaN is coupled with Ti, the GaN for lack of N has to secure sufficient carriers having high density. Accordingly, current flows well by the decrease of resistance.
Therefore, while an n type electrode is made of Au/Ti, Al/Ti, or Au/Ti/Al/Ti, a p type electrode is made of Au/Ni/Cr. The reason that a stack of metal layers is used as an electrode is because of morphology. When a material for electrode is selected, its reaction with GaN as well as morphology of the electrode should be taken into account.
Unless the surface of an electrode is clean, wires in connection with the electrode may be disconnected, thereby deteriorating device performance.
Therefore, a plurality of metal layers, stacked to form an electrode, should have good reaction with one another, thereby reducing the morphology of the electrode. However, morphology is still a problem.
A background LED has the following problems. Since many metal layers are stacked to form an electrode and then annealed at a high temperature of higher than 700.degree. C., the whole fabricating process is complex and inefficient. Further, when the annealing process is performed, Ti in the electrode is diffused and finally coupled with Al or Au. As a result, the surface of the electrtode is fluctuated and its reaction with GaN becomes poor so that it becomes impossible to obtain a desired resistance. Furthermore, it is difficult to perform following process steps such as wire bonding due to morphology.